A variety of air purification systems have been developed to protect people from hazardous air contaminants. Among these air purification systems are a wide range of air purifying respirators that are designed to filter out or adsorb contaminants present in the air. Typically these air purifying respirators contain a filter media, a filter body, or some combination of filter media and filter body. Upon use of the respirator, the contaminants become absorbed by the filter media or attached or trapped by the filter body. Eventually, the filter media or filter body becomes saturated and the ability of the respirator to remove the hazardous air contaminants begins to diminish.
During extended exposure to an environment containing hazardous air contaminants, such as, for example, continuous or repeated worker exposure to such environments, techniques are necessary to determine the useful service life of a respirator. One technique that has been developed is based upon the time in service for a respirator. In this technique, respirators or the air purifying filters are replaced after a certain period of time in service, based upon, for example, a mathematical model such as that of Wood et al. in the Journal of the American Industrial Hygiene Association, Volume 55(1), pages 11-15, (1994). However, this technique does not take into account variations in contaminant level or flow rates through the respirator and therefore may result in the respirator or filter elements being changed too early (which is wasteful) or too late (which may present a danger to the user).
Examples of filter cartridges that contain layers or mixtures of different sorbent materials include U.S. Pat. No. 5,660,173 (Newton) which describes a cylindrical canister for use in conjunction with a gas mask that includes a frustum shaped carbon bed and a layered array of different size carbon particles in the carbon bed. U.S. Pat. No. 5,714,126 (Frund), describes a respirator filter system for filtering toxic agents that includes a cartridge containing a layer of an unimpregnated activated carbon, a layer of an activated carbon impregnated with sulfate, molybdenum and copper or zinc, and a HEPA filter. U.S. Pat. No. 6,344,071 (Smith et al.) describes filter media that includes at least two kinds of filter media, a first plurality of filter media particles containing a transition metal impregnant and a second plurality of filter media particles containing a tertiary amine impregnant.
A wide variety of End of Service Life Indicators (ESLI) have been developed for use with the filter cartridges of respirators. Generally, the ESLIs are described as being passive or active. Passive ESLIs are ones where a change in the indicator (often a color change) is caused by exposure to an analyte when a sorbent for that analyte is near depletion. Active ESLIs are ones that incorporate an electronic sensor to monitor a gas stream for an analyte and upon detection of the analyte because of sorbent depletion, to produce a warning signal.